I. Field of the Invention
This invention is related to novel inverse emulsion cross-linked polymer of controlled pH obtained from partly neutralized acrylic acid by use of siloxane-emulsifier system for water-in-oil emulsion polymerization. It further relates to a method for carrying out an inverse emulsion polymerization, and product obtained therefrom.
II. Description of the Prior Art
A variety of water-soluble polymers are produced by precipitation polymerization. Typical of such polymers are cross-linked polyacrylic acids, such as, the Carbopol.RTM. resins of B.F. Goodrich, which have been widely used as thickeners for aqueous systems. The unique thickening efficiency and rheological properties of these resins are attributed to the incorporation of an allyl cross-linker, such as, polyallyl sucrose or polyallyl pentaerythritol. With this technique, the polymer is usually produced in an organic solvent, e.g., benzene or ethyl acetate, from which it precipitates and is recovered in the form of a powder, which has to be filtered and dried. This presents a number of problems. For example, often residual organic solvent as well as unreacted monomer may be retained in the separated resin particles. For most end uses, these residual materials cannot be tolerated. In particular, these polymers are often used as thickeners in personal care products. The presence of such toxic solvents or monomers renders the polymer unusable for this purpose, unless it is first treated to remove the impurities. Such treatments not only increase processing time, but significantly increase the processing cost.
Additionally, because the polymeric product is obtained in particulate form, and since it is normally utilized because of its ability to swell in water, the particles must again be redispersed into the end use composition. Such mixing and redispersion generally require rather complex mixing equipment which often is not available at the site of end use.
Additionally, the polymer is obtained in acid form and must be neutralized before it can be used in a personal care product. The neutralization step is required to obtain the thickening properties of the polymer. This again requires another processing step, adding to the cost of the use of the product.
Another important process for making water-swellable cross-linked polymers on a production scale is inverse emulsion polymerization.
In a typical inverse emulsion polymerization, a water-in-oil monomer emulsion is produced by adding a water phase of an aqueous solution of monomers, to an oil phase consisting of mineral oil, aromatic or aliphatic hydrocarbons and the like. The oil phase usually contains an appropriate emulsifier. The monomer emulsion is then homogenized to obtain the proper particle size and then subjected to polymerization conditions to form a water-in-oil polymer emulsion, using a water-soluble or water-insoluble initiator. This inverse emulsion polymerization process has the advantages of producing polymers of a high molecular weight at a high polymerization rate compared to polymer from a typical solution or precipitation polymerization process of the same monomer. If a cross-linked polymer is to be obtained, a water-soluble cross-linking monomer is added to the system. Thereafter, a high HLB value surfactant is incorporated into the polymer emulsion and water is added to convert it into an oil-in-water system.
Although the inverse emulsion polymerization process has the above noted advantages, many problems, such as, poor emulsion stability, incomplete polymerization and difficulty in inverting into an oil-in-water system remain to be solved. One of the problems is obtaining a homogeneous, stable initial water-in-oil emulsion, containing monomer in water phase. This aim is usually achieved by using complex and expensive equipment, e.g., homogenizers, high shear mixers, and the like.
A variety of emulsifiers (surfactants or surface active agents) are used for the emulsification, sorbitan monooleate being one of the most popular and readily available. While these form stable emulsions, the water-in-oil polymer emulsion has typically a very high bulk viscosity and is difficult to be inverted into oil-in-water system.
United States patents relating to the foregoing include:
U.S. Pat. No. 3,826,771 discloses the use of sorbitan monooleate and sorbitan monostearate as the water-in-oil emulsifying agents.
U.S. Pat. No. 3,278,506 discloses the use of ethylene oxide condensates of fatty acid amides as the emulsifiers.
U.S. Pat. No. 4,024,097 discloses the use of surfactant systems consisting of an oil-soluble alkanolamide (e.g. Witcamide 511, Witco Chemical) and one or more co-emulsifiers of unesterified dialkanol fatty amide, salts of fatty tertiary amines, quaternized ammonium salts of fatty tertiary amines, alkali metal salts of fatty acids, and alkyl or alkyl aryl sulfates or sulfonates.
U.S. Pat. No. 4,672,090 and 4,772,659 both disclose a surfactant system for water-in-oil emulsions comprising an oil-soluble alkanolamide, a polyoxyethylene derivative of a sorbitan ester, and sorbitan monooleate.
One method for avoiding the above noted problems in inverse emulsion polymerization processes has been proposed utilizing an emulsifier which has an HLB value no greater than 7 and polymerizing a water-soluble monomer of a water-soluble polymer in a water phase present in a water-in-oil emulsion and in the presence of an emulsifier comprising a mixture of a non-ionic oil soluble surfactant and a compound which may be a N-alkyl lactam or polymers of alkylated N-vinyl lactams. This emulsifier is disclosed in a copending application Ser. No. 07/760,272, filed Sep. 16, 1991, and entitled LACTAM-CONTAINING EMULSIFIER SYSTEMS FOR WATER-IN-OIL EMULSION POLYMERS naming Jui-Chang Chuang as the inventor, a co-inventor of the present invention.
The emulsifier and the process disclosed in this copending application are particularly useful for producing easily inverted oil-in-water emulsions of polymers which are suitable for use in the personal care area, for example, polyacrylates. However, certain problems arise in connection with this process and the product obtained therefrom specifically, when the monomer is acidic, the water-in-oil monomer emulsion is not stable.
The above problems are particularly difficult to solve in the case of the polymerization of an unsaturated carboxylic acid monomer e.g., acrylic acid. Unneutralized acrylic acid dissolves in both water and oil phases, and thus makes it impossible to carry the inverse emulsion polymerization. In contrast, acrylic acid salts such as, sodium, potassium or ammonium acrylates are soluble exclusively in the aqueous phase, which allows the inverse emulsion polymerization to be easily carried out. However, the polymerization of fully neutralized acrylic acid i.e., having a pH in the water phase of about 7, generates poly(sodium acrylate) which, in diluted water solution, has a pH of about 9. The same principle applies to lightly crosslinked poly(sodium acrylate). The high pH of polyacrylate polymers prepared from fully neutralized acrylic acid monomer limits their applications, particularly in the skin care area, which requires a pH in the range of about 6 to 7.5.
Although it is possible to adjust the pH of such crosslinked poly (sodium acrylate) solution with an acid, e.g., acetic or citric acid, such a pH adjustment produces a corresponding sodium salt, which, in turn, substantially reduces the thickening power of the polymer.
An attempt to polymerize partially neutralized acrylic acid monomer in an effort to produce a resulting polymer having the required pH of 6 to 7.5 in diluted aqueous solution by known methods was unsuccessful. This was due to the inadequate stability of water-in-oil monomer system containing the partially neutralized acrylic acid in the aqueous phase. We have found that none of the conventional surfactants used in an inverse emulsion polymerization according to prior art were sufficiently efficient to make polymerization of such a system possible.